A Global Navigation Satellite System (GNSS), such as the Global Positioning System (GPS), typically comprises a constellation of navigation satellites in Medium Earth Orbit, a ground Control Segment that controls these satellites, and user equipment (UE). The navigation satellites broadcast precisely synchronized ranging signals. The GNSS UE acquire and lock on to the signals from multiple satellites, make precise ranging measurements to the satellites, apply corrections, and apply navigation filter algorithms to solve for the UE position, velocity and time (PVT).
GNSS UE can also operate in the space environment, such as on board satellites. The space environment often creates additional challenges to GNSS UE operation compared to terrestrial UE applications, including: continuous and wide ranges of platform rotational motions, intermittent antenna visibility to individual GNSS satellites, reduced availability of GNSS signals, weaker received GNSS signals, high relative velocities and Doppler effects, larger relativistic effects, larger signal dynamic range, and increased sensitivity to timing errors.
GPS/INS ultra-tight coupling (UTC) is a methodology of integrating GPS and inertial navigation system (INS) instruments to enhance UE robustness and anti-jam (AJ) performance. In a GPS/INS UTC implementation, all of the GPS correlator signal measurements associated with each pseudorandom (PRN) code ranging signal are provided to a UE error state Kalman navigation filter. The Kalman filter provides updates to the UE Position-Velocity-Time (PVT) state errors, and the GPS signal tracking loops are closed through the INS and Kalman filter processing.
The GPS Modernized Space Receiver (MSR) program is aimed at developing the next generation space receiver which will be capable of operating with legacy civilian coarse acquisition code (C/A-code), L2 civil code (L2C), and military encrypted precision code (P(Y)-code) GPS signals as well as the new military code (M-code), L5 civil code, and L1 civil code (L1C) GPS signals. Such a receiver must also be able to operate in the space environment for the design life of the host satellite (e.g., ten or more years), have some anti-jam capabilities and meet certain objective tracking requirements. UTC has been proposed as an MSR architecture solution.
U.S. Pat. No. 6,516,021 describes a GPS/INS ultra-tight coupling UE and assumes the use of a complete INS that includes both translational and rotational motion sensors as needed in order to sense and predict the motion of terrestrial based platforms whose acceleration or jerk and rotation are unpredictable.
The use in GPS/INS UTC space borne UE of inertial measurement unit (IMU) accelerometer instruments, such as for sensing translational motion, entails substantial costs, including the cost of space qualifying the associated electronics.
Conventional non-UTC GPS UE which implement random motion models for acceleration or jerk can introduce significant error for rotating satellites, especially if no correction is applied to relate the satellite center of gravity (CG) location to the GPS solution as computed for measurements made at the GPS antenna.
Conventional GPS UTC IMU implementations, such as described in U.S. Pat. No. 6,516,021 compute a PVT solution at the location of the IMU, which is generally not at the platform CG location. The traditional UTC antenna lever arm (ALA) correction relates the GPS antenna phase center (APC) location to the IMU location. For satellite applications, however, the PVT solution of the satellite CG trajectory is required for precise orbit determination applications.
U.S. Pat. No. 5,909,381 describes the use of on-board force models to predict GPS satellite orbital CG motion. The subject matter of U.S. Pat. No. 5,909,381, however, does not attempt to correct the satellite orbit prediction using state error models with GPS receiver measurements of L-band ranging signals broadcast by the GPS satellites. In addition, U.S. Pat. No. 5,909,381 is specific to GPS satellite applications. The use of GPS receivers on GPS satellites is generally not practical because of the jamming of the receiver due to the proximity of the satellite transmitter and GPS radio frequency (RF) signal radiator.